The object of the present invention is to provide a violet-colored quinacridone pigment of high saturation which has the heat stability required for utility as a pigment in engineering plastics. This object is achieved by preparing violet pigment compositions comprising a 2,9-dichloroquinacridone pigment in combination with a sulfonic acid or a sulfonic acid salt of a blue-colored indanthrone or copper phthalocyanine.
Quinacridones are valuable pigments. The beta crystal modification of the unsubstituted quinacridone, C.I. Pigment Violet 19, is known for its attractive violet color. Although it is of coloristic interest, the violet-colored beta-phase of unsubstituted quinacridone possesses insufficient heat stability to have utility for pigmenting the engineering plastics.
Since 2,9-dichloroquinacridone having a specific surface area below 30 m.sup.2 /g has good heat stability, it is useful for pigmenting engineering plastics. However, due to its large particle size, the color is shifted to a unique red color shade from the known magenta color of smaller particle size 2,9-dichloroquinacridone. Thus, no violet colored quinacridone pigment is known to possess the heat-stability required for utility as a pigment for the engineering plastics.
Indanthrone and copper phthalocyanine are known in the art as blue pigments which possess good heat stability. The sulfonic acid derivatives of indanthrone and copper phthalocyanine are known as dyestuffs and as intermediates in the preparation of dyestuffs.
The present invention is based on the discovery that violet pigment compositions comprising a 2,9-dichloroquinacridone pigment in combination with a sulfonic acid or a sulfonic acid salt of a blue-colored indanthrone or copper phthalocyanine possess high saturation and the heat stability required for utility as a pigment in engineering plastics. In addition, the inventive pigment compositions are highly dispersible in the plastic medium to be pigmented. Thus, the inventive compositions provide a violet quinacridone pigment which is suitable for the utility of pigmenting engineering plastics.
Although violet mixtures of pigmentary indanthrone or copper phthalocyanine with pigmentary 2,9-dichloroquinacridone can be prepared, a larger amount of the blue pigment is needed in order to achieve the desired violet color. Because of the larger amount of the blue pigment required, the saturation of these pigment mixtures is too low. In addition, these pigment mixtures are not as homogeneously dispersed in the plastic medium as the present compositions.